Clipping Groups
Having created one or more Adjustment
Layer, it may not be convenient for it to cause changes to all
the layers beneath. Photoshop will allow you to bind one or more Adjustment
Layers to the required image layer by 'clipping' the Adjustment Layer
to it.

This is how you do it.

Create your Adjustment Layer
and click 'OK'.

Activate either the newly
created Adjustment Layer or the layer below, then, whilst holding down
the Alt key, move the cursor over the junction between the two Layers
('Adjustment' Layer above and 'image' layer below). As you slowly cross
the dividing line, the cursor will change from a that of a 'hand' to small
grey/white 'eclipse' symbol

the 'Clipping Group' ('eclipse') symbol

When you see the 'Clipping Group'
symbol appear (above), click (mouse) or press (Wacom pen) on the dividing
line between the two layers line and the two adjustment layer thumbnails
will displace to the right and a downward arrow will appear upon
the layer, indicating that it is 'clipped' (associated) to the layer beneath.
In earlier versions of Photoshop, the dividing line between the two layers
changes from a solid line to a dotted line.

Left: Clipping Group - the
upper layer is 'clipped' to the lower.

The two Layers now form a
'Clipping Group' and adjustments made will only affect the image layer
to which it is 'clipped'.

To 'unclip' the two layers,
repeat the above procedure and they detach from one another.